I think, long term the proc rate is somewhere around 15-20%.
So an average of one extra Truegold per five transmutes might be correct.

My first transmute, resulted in 10 bars across the five toons.
But I've had days where five toons get five bars.

And at the moment, only the one team has the Truegold transmute.

The most profit has been, transmuting with their mats for a 200-300g transmute fee.
Having all five of my toons grouped together, and just blow out as many of these as possible in a short time frame.
And then mailing any "procs" to my auction toon, as a free profit.

I think the Air is a safer transmute.
The Life will be a lower investment (5g per)...
And there is more demand for Air (it has held at 60g, on my server), while Truegold has fallen from 1800g to 900g, which is a loss compared to the cost of making a single bar at AH prices...
While your prices might be lower, through transmuting Life into Fire/Water/Air, if the mats are worth more than the Transmuted item, doing the transmute is a loss over just selling the mats.